The present invention relates to improved hydraulic fluids useful in applications where the properties of existing hydraulic fluids present a significant fire hazard due to their inherent flammability. Typical applications for such non-flammable fluids include various civilian and military aircraft, tanks and transport vehicles in which hydraulic systems are subject to damage or failure resulting in leakage and ignition of the fluid.
Existing hyradulic fluids commonly used today in such applications are mineral, naphthenic, or synthetic oils which have been selected primarily on the basis of hydraulic properties without particular regard for non-flammability requirements. In fact, these fluids tend to be highly flammable and cannot be rendered non-flammable by the use of additives or special processing. Typical hydraulic oils which are used extensively in military vehicles are oils designated as MIL-H-6083 and MIL-H-46170.
Among the synthetic oils which have acceptable hydraulic properties, and which are also commercially available, are the chlorotrifluoroethylene-derived oils (hereinafter "CTFE" oils). These oils are essentially non-flammable due to their high degree of halogenation and can thus be used in hydraulic applications where the non-reactivity of the fluid is an essential requirement. CTFE oils are saturated, low molecular weight polymers (i.e. telomers) of chlorotrifluoroethylene, typically having from about 2 to about 10 repeating units in the telomer chain. The terminal groups of the telomer chain are generally derived from the catalyst and/or the solvent used in the telomerization process. The chemical and thermal stability of such CTFE oils is enhanced by fluorination of the terminal groups of the telomer.
A further requirement of hydraulic fluids is compatability with the particular sealing or gasket materials used in the hydraulic system. Sealing materials commonly used in such applications are generally in the form of O-rings fabricated from synthetic elastomers, such as acrylonitrile-butadiene rubber, fluoroelastomers, and the like. Frequently more than one type of elastomer is used in a hydraulic system for a particular piece of equipment. For example, various military hardware currently in use by the U.S. Army employ hydraulic seals fabricated from acrylonitrile-butadiene rubber, hereinafter "nitrile" rubber, and fluoroelastomeric seals such as Viton, a fluoroelastomer based on a copolymer of vinylidene fluoride and hexafluoropropylene manufactured and sold by the E. I. duPont de Nemours & Co., and PNF, a fluorophosphazene fluoroelastomer manufactured and sold by the Firestone Tire and Rubber Co.
In general, the seals for a particular piece of equipment are initially designed and selected on the basis of the specified hydraulic fluid. Thus, as would be expected, conventional hydraulic fluids used in such military hardware, i.e. MIL-H-6083 and MIL-H-46170, are compatible with seals of this type over an operating temperature range of from about -65.degree. F. to about 250.degree. F. However, the CTFE oils, although generally compatible with fluoroelastomers, have been found to be incompatible with nitrile rubber seals due to shrinkage and embrittlement of these seals in a relatively short period of time in the presence of CTFE oils. This shrinkage and embrittlement can result in leakage of fluid from the hydraulic line and consequent failure of the hydraulic circuit.
Elastomers employed as seals in hydraulic systems should swell slightly upon contact with the hydraulic fluid to provide a fluid-tight seal over the operating temperature range of the equipment. An acceptable range of swelling for purposes of this invention on the basis of percent relative volumetric expansion is generally from about 5% to about 15% over a temperature range of from about -65.degree. F. to about 250.degree. F. Of course, as implied from this statement, any shrinkage of the seal during operation of the hydraulic system cannot be tolerated since this will result in leakage of fluid from the system. Over-expansion of the seal and excessive softening will also lead to leakage of fluid.
It is also desirable for the seals to retain the same physical properties such as tensile and elongation strength after prolonged exposure to the hydraulic fluid. Therefore, the fluid should contain components which are capable of at least partial replacement of material extracted from the seals.
In addition to the foregoing, trace amounts of copper or copper-bearing metals such as naval brass from hydraulic system components can be present in the hydraulic fluid. The presence of even trace amounts of such metals can also cause hardening and embrittlement of the seals.
Commonly assigned copending application Ser. No. 519,940, filed Aug. 3, 1983, describes a non-flammable hydraulic fluid which is compatible with both nitrile rubber seals and fluoroelastomeric seals. This fluid comprises a fluorinated chlorotrifluoroethylene oil, an aliphatic ester, a liquid polyester or polyether, a dewaxed hydrocarbon oil, and a phenolic antioxidant. Unfortunately, however, this fluid will not prevent hardening and embrittlement of nitrile rubber seals in the presence of copper or copper-bearing metals.
Accordingly, it is a primary object of the present invention to provide a non-flammable hydraulic fluid which is compatible with both the seals and metal components used in hydraulic systems.